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LA Audio 4C

Quad Compressor Limiter By Paul White
Published February 1994

Whenever LA Audio design a new processor, they seem to take the Swiss Army Knife approach, kitting it out with a host of practical add‑ons to increase its usefulness. So Paul White wasn't too surprised to find that the 4c is more than just a quad compressor...

LA Audio seem intent on combining value, quality and versatility, which is good news for their customers if not for their competition. Their 4x4 proved very popular, with its combination of two gates, two compressors and two sets of assignable side‑chain filters in a 1U box, but the 4c goes even further. The new processor comprises four independent compressors, each with its own filter, but it is the number of different ways in which the filter can be used that makes the system so clever. The four channels may be linked to operate as two stereo pairs, as would be required for processing stereo mixes, subgroups or instruments, but a new twist is the way in which all four channels are 'normalised' such that all four compressors are wired in series if no rear‑panel connections are made. Why exactly you might want to compress the same signal four times in one go will be revealed a little later.

Once the individual compressors are patched up, the chain is broken and independence is restored. As all four channels are identical, a description of a single channel will tell most of story.

The Compressor

Each compressor has its own input and output on electronically‑balanced jacks which can accept either balanced or unbalanced signals in the range ‑10dBV to +4dBu. A further pair of jacks provide access to the side‑chain input and output points so that additional processors — equalisers,for example — may be patched in or, alternatively, an external signal may be fed into the side‑chain input to convert the compressor to a ducker. If nothing is plugged in, the compressor action is controlled by the input signal as usual.

The compressor circuit combines a soft‑knee action with a conventional Ratio control (the same approach as used on the LA Classic Compressor and 4x4). The soft‑knee forces the compression ratio to increase gradually around the threshold point rather than simply switching in all the gain reduction as soon as the input reaches the threshold value, and this provides more natural‑sounding control, especially on complex mixes. The compressor also adds a little controlled distortion during periods of very high compression which makes it behave rather like an exciter, the idea being that this helps compensate for any brightness lost due to heavy compression.

The compressor controls are quite conventional: Threshold, Ratio, Release and Gain. There's no variable attack control; instead there's a switch which either sets a very fast attack time or a slower setting, whichever is most appropriate. Release is fully variable from 20mS to 4S and the Gain control provides up to 24dB of make‑up gain to compensate for any gain lost during compression. The bypass switch links the input to the output so the signal can still get through, even with the power off — but as far as I can tell, this is only true if you're using balanced signals. With unbalanced leads, I found that Bypass only worked properly with the power left on.

A 10‑segment meter monitors the gain reduction at all times, and in bypass mode this continues to function but at a reduced intensity so that you know what degree of compression to expect when you switch the compressor back in. Because the Ratio control goes from 1:1 to 20:1, the 4c can go all the way from the gentlest compression to hard limiting. In all, a comprehensive and flexible compressor section, but the Swiss Army Knife doesn't become evident until we move onto the filter section.

The Filter

The Filter section is where it all starts to get rather clever. All you have is one knob and two buttons, but they hold the key to the less orthodox operating modes of the 4c. Pressing the HP/LP button switches the filter from high‑pass to low‑pass operation, while the Filter control sets the filter's cutoff frequency in the range 100Hz to 10kHz. The filter is always in the side‑chain circuit so if you don't want to use it, you have to set it either to 10Khz low‑pass or 100Hz high‑pass to prevent it having an effect on the side‑chain signal. This works fine, but it's easy to forget about the filter setting if you're not actually using it; a Filter Out switch would have been more useful. For conventional de‑essing, the filter would be set at around 2kHz HP whereas de‑popping might require a setting of 200Hz LP or thereabouts.

All the above operations come under the category of full‑band compression: no matter from what part of the audio spectrum the side‑chain is triggered, gain reduction is applied to the whole signal. For example, when de‑essing, the compressor is most sensitive to the frequencies above 2kHz, but during sibilant sounds,the whole input signal is compressed, not just the bit above 2kHz. For this reason, de‑essing has to be set up carefully in order to avoid over‑processing the signal to the point that the compression becomes obvious. In theory, you could make a compressor that only acted on one part of the audio spectrum while leaving the rest alone — and that's where the other switch comes in.

Half Band

With the Half Band switch pushed in, the internal signal flow of the compressor is changed so that the filter acts like an electronic crossover, splitting the signal into two frequency bands at a point determined by the Filter setting. In other words, if you were to set the filter to 1kHz, the audio would be split so that one path contained only frequencies below 1kHz and the other path only frequencies above 1kHz. If we were to compress only one part of the signal before recombining the two parts of the spectrum, we would have a simple way of compressing just a part of the audio range while leaving the rest untouched — and that's exactly what the 4c does in this mode. Depending on the setting of the HP/LP switch, it's possible to compress either those frequencies falling below the crossover point or those falling above it, so for improved de‑essing, you'd set the HP/LP switch to its out position, push the Half Band switch in and set the filter to around 2kHz, depending on the voice being treated. Now the compressor deals only with those frequencies above 2kHz and leaves the low and mid frequencies untouched. In theory, this should be far less obtrusive than full‑band de‑essing. In Half Band mode, the Gain control affects only that part of the signal that is being compressed, so there's no problem achieving a good tonal balance.


The final mode of operation requires two or more channels to be cascaded together, a task handled by the normalising jacks when no connections are made other than to the input to the first compressor in line and the output from the last compressor in line. If each band is configured as a half‑band compressor with the HP/LP switch out, and the filter controls set at different frequencies, with the first channel being the highest frequency and so on, different sections of the audio spectrum can be set to be compressed by different amounts. The upper and lower limits of each band are fixed by the filter settings of adjacent channels.

The review unit spent most of its time in Half Band mode, as I feel that this provides the most creative opportunities both in the treatment of single sounds and complete mixes.

With all the compressors cascaded, the audio spectrum could be split into five frequency bands, one of which would be uncompressed and the rest of which could be compressed by independent amounts. This is really experimental territory and I feel the facility was included only because the existing features made it easy to do. However, selective compression can be used to improve awkward vocal sounds, or to tame trouble spots in complex mixes.

In Practice

As a straight compressor, the 4c behaves much like the 4x4 — use it gently and it keeps your levels under control in a polite and efficient way, but lay it on more thickly and the sound starts to pump slightly, adding a touch of energy and excitement. Most compressors can either be one thing or the other, but this one makes a good crack at both. My favourite mode, however, is Half Band, not just for de‑essing, but for changing the spectral dynamic of sounds or entire mixes. For example, set the compression to affect just what's going on over 5kHz, set the threshold to give between 10 and 20dB of gain reduction, and then use the Gain control to add just the right amount of compressed top end to the mix. Back off the gain and you can tame an over‑bright mix; advance it and you get an effect rather like an exciter. Do the same trick at the bottom end of the spectrum and you can take the boom out of an over‑bassy mix or tighten up what's there and use the Gain control to add more of it.

Multi‑band mode demands a little more patience, and because you can mix HP and LP filter settings, there are several different ways in which you can define your bands. However, with all four sections cascaded, I found it hard to tell which control was doing what — the whole thing becomes rather more controllable if you simply use two pairs of two compressors. This enables a stereo mix to be processed and still provides plenty of control; with a little care, you can now enhance both the top and bottom end of a mix, leaving the mid‑band unchanged.


This is definitely much more fun than your average compressor, yet if straightforward compression is what you're after, the 4c can handle that too. I must admit, though, that the review unit spent most of its time in Half Band mode, as I feel that this provides the most creative opportunities both in the treatment of single sounds and complete mixes.

And now the obligatory whinges. You might expect me to criticise the lack of an attack control, but the two switch settings provided seemed to cover most eventualities without serious compromise. No, that's not what worries me: what makes like unnecessarily hard is a simple matter of front‑panel legending — there is no way to see what function is selected when the buttons are in or out because there are none of those little graphic icons to show you. You're told that the HP/LP button switches between high‑pass and low‑pass, but without referring to the manual, I can't remember which is in and which is out. The same is true of the Half switch and the Attack switch. I've communicated this grievance to the chaps at LA Audio and they appreciate my concern, so perhaps they'll modify the screen print before the next production batch hits the shops.

In all other respects, and given the very attractive price of what is actually a very professional piece of kit, the 4c gets a definite thumbs up. If you already have a favourite compressor, the 4c would make a good second model; it has a good level of basic performance plus you can use it to do a few weird things that a conventional compressor might not handle. It also works well as a ducker or de‑esser. For those who don't have a compressor, the 4c is good enough to use as a main compressor/limiter in most applications, yet can still turn its hand to the more esoteric jobs made possible by its comprehensive filter switching system. It's particularly encouraging to see British designers coming up with products that not only beat the far eastern manufacturers in terms of price, but also out‑innovate them into the bargain.

4C Filter Modes

  • FULL BAND COMPRESSION: this mode describes conventional compression or limiting and also simple de‑essing. When de‑essing, the filter is set in HP mode so that the compressor reacts mainly to frequencies above the filter's set frequency. However, gain reduction is applied to the whole signal, regardless of the filter setting. Ducking may be achived by feeding an external line‑level signal into the side‑chain input socket.
  • HALF BAND COMPRESSION: in this mode, the filter acts as an electronic crossover, splitting the audio into two bands. In HP mode, only the frequencies above the filter frequency are compressed; in LP mode, only frequencies below the filter setting are affected. The two audio bands are recombined at the output, the Gain control providing a means to balance the compressed portion of the signal with the uncompressed portion. This mode provides a less obtrusive means of de‑essing and de‑popping, but may also be used to produce either treble or bass enhancement.
  • MULTI‑BAND COMPRESSION: using the normalised in/out jacks on the rear of the unit, two or more channels may be cascaded, and each one may be set to treat a different part of the audio spectrum. Both high and low pass filtering may be combined, but the number of possibilities means you really need to work out what you're trying to do in advance of setting up the unit. I found using two channels of two‑band compression to be sufficiently flexible without being too difficult to set up. This proved useful for treating a stereo mix, where it may be used to enhance the top and bottom end simultaneously.


  • Extremely versatile compressor/limiter.
  • Useful extra functions not found on most compressors.
  • Competitive price.


  • Panel legending doesn't make the switch operation as clear as it might be.
  • Working in multi‑band mode can be confusing.


A really flexible and good sounding compressor that is priced to appeal to the home user but proficient enough for serious professional use.